The present invention relates to a combined tool for precision boring and burnishing tubular workpieces.
Inner surfaces of bores requiring a high surface quality are mostly produced in three steps, that is preboring, finish boring, and honing.
Since honing is a very time consuming method attempts have been made to replace this honing step by a less time-consuming one.
The honing step has therefore been replaced by a step of smooth rolling or burnishing the inner surface of a tubular workpiece. In this way, the manufacture of high quality bores, as for instance required for cylinder bores of pneumatic or hydraulic cylinders, has been considerably reduced in cost. In principles, one can therefore divide the manufacture of such bores into a material removing step and a non-cutting finishing step.
The German patent 958,896 discloses a combined tool for carrying out these two steps in a single operation. Thereby it is assumed that the workpieces which are to be machined with such a combined tool are already provided with an internal bore, that is that the above-mentioned first step of preboring is already carried out before the tool is used.
Further developed tools of the aforementioned kind are disclosed in the German Offenlegungsschrift No. 2,209,234.
In these known tools, the boring head is rigidly connected with the trailing burnishing head. This combined tool is, for instance, during use in a known boring machine, rigidly connected with the boring bar. The tubular workpiece to be machined is mounted between two cones of a common boring machine and rotated by means of a driver. The precision boring head is provided with guide bars which abut against the inner surface of the bore formed in the cylinder to be machined. When the combined tool is advanced by the boring bar, the knives on the boring head will machine the preformed cylinder bore to a predetermined inner diameter. During further advance of the combined tool by means of the boring bar, the rollers of the burnishing head will engage the inner cylindrical surface of the workpiece and roll this surface smooth. The problem of properly removing the shavings produced by the combined tool is solved in accordance with the teachings of the mentioned German Offenlegungsschrift No. 2,209,234.
While properly removing of the chips or borings formed by the combined tool does not provide any difficulties any longer, the aforementioned described combined tool is, while being located in the bore to be machined, statically overdefined, which results in a considerable wear on the guide bars of the boring head. In addition, the thus-constructed tool is subjected to oscillations which leave marks on the surface to be machined, which are not completely eliminated by the subsequent rolling operation.
The combined precision boring and burnishing tool in which the boring head and the burnishing head are rigidly connected to each other is guided in the bore to be machined on the one hand by the guide bars of the boring head and on the other hand by the rolls of the burnishing head. The combined tool is, however, also rigidly connected with the boring bar which, on the one hand, is supported and guided at a portion distant from the tool in the stuffing box of an apparatus for feeding boring oil into the hollow bore bar and, on the other hand, by its connection with the combined tool through the rolls of the burnishing head. Due to misalignment between the axis of the workpiece and the axis of the machine tool on the one hand, and due to the considerable weight of the boring bar on the other hand, the latter, thus supported between two points may be bent in any plane. The connection between the combined tool and the boring bar is then, when the combined tool is located in the bore to be machined, subjected to bending and shearing forces. This will result in that the combined tool, located in the bore to be machined, will also be bent, in a direction opposite to the bending of the boring bar. This in turn will result into an undesired tilting of the cutting knives of the boring head and in addition in undesired change of the tearshaped impressions which the rolls of the burnishing head produce on the surface to be finished. The radial forces which the workpiece produces in the tool during bending of the combined boring and burnishing tool will also lead to a premature wear on the guide bars and on the burnishing rolls. The thus-produced force by the bent boring rod and by the bent tool onto the workpiece will often result in rifling and chattering conditions which cannot be properly suppressed.
A further tool for precision boring and burnishing of tubular workpieces is disclosed in the German Gebrauchsmuster No. 7,344,800. In this known tool, the inner surface of the workpiece is precision bored while the combined tool advances into the bore to be machined and, during the subsequent withdrawal of the combined tool, in which the knives on the boring head are removed, the thus-precision bored inner surface is rolled smoothly by the rolls of the burnishing tool. During the advance of the combined tool, the burnishing rollers are not in contact with the workpiece and during retraction of the tool, the knives of the burnishing head, which have been removed, are evidently not in contact with the workpiece. During operation of this tool, there will not arise a statically overdefined situation, and while the combined tool may tilt with respect to the axis of the tubular workpiece, such tilting will not produce any detrimental effects.
A disadvantage of the last-mentioned tool, as compared with the first-described tools is, that the necessary time for finishing the workpiece is considerably increased. Whereas in the first-described tools, the bore to be machined is precision bored and smoothly rolled during advance of the tool into the workpiece and the withdrawal of the tool can therefore be produced at considerably greater speed, in the last-mentioned tool the speed at which the workpiece has to be retracted has to be the same as the speed in which the workpiece is advanced, which evidently increases the necessary time for finishing the workpiece.
Therefore, the last-mentioned tool has not found great use in the art.